The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. The display usually comprises two plates with electrodes placed opposing each other, separated by spacers. One of the electrodes is usually transparent. A suspension composed of a colored solvent and charged pigment particles is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side or the other, according to the polarity of the voltage difference. As a result, either the color of the pigment particles or the color of the solvent is seen from the viewing side. Alternatively, the suspension may comprise a clear solvent and two types of colored particles which migrate to opposite sides of the device when a voltage is applied. Further alternatively, the suspension may comprise a dyed solvent and two types of colored particles which alternate to different sides of the device. In addition, in-plane switching structures have been shown where the particles may migrate in a planar direction to produce different color options.
There are several different types of EPDs, such as the conventional type EPD, the microcapsule-based EPD or the EPD with electrophoretic cells that are formed from parallel line reservoirs. EPDs comprising closed cells formed from microcups filled with an electrophoretic fluid and sealed with a polymeric sealing layer is disclosed in U.S. Pat. No. 6,930,818, the entire contents of which are hereby incorporated by reference as if fully set forth herein.
There are many ways to switch the image on an electrophoretic display from one image to another that use direct transitions from one to the other and bipolar driving. Driving method may involve writing of the first image to a uniform dark or white state and then to the second image, writing the first image to a uniform white state then a dark state and then to the second image, cycling the dark to white image many times before writing the second image, writing complex checkerboard patterns between images, and so forth. The purposes of such complex waveforms are to prevent residual images by ensuring full erasure of one image before writing the other.
However, there are many characteristics of prior waveforms which will cause image degradation. Residual image poor bistability, improper grey level setting, changes in performance with time, temperature, and light and so forth are many known problems that current waveforms cause when used to write an electrophoretic display.